1. Field of the Invention:
The present invention relates to magnetic recording mediums which make use of thermal assistance for recording information.
2. Description of the Related Art:
Magnetic recording mediums, such as magnetic discs, are known as recording means used for storage devices such as hard discs. Magnetic discs have a laminate structure which includes a disc substrate and a recording layer having a predetermined magnetic structure. With increasing amount of information processing in computer systems, there is a demand for magnetic discs capable of recording at a higher density.
When recording information in a magnetic disc, a recording magnetic head is placed closely to a recording surface (provided by the recording layer) of the magnetic disc. The magnetic head applies to the recording layer a recording magnetic field which is stronger than the coercive force of the recording layer. While the magnetic head is moved relatively to the magnetic disc, the direction of the recording magnetic field applied by the magnetic head is inverted successively, whereby there is formed in the recording layer a plurality of recording marks (magnetic domains) whose direction of magnetization is inverted successively in a circumferential direction i.e. in a track running direction. By controlling the timing of inverting the direction of recording magnetic field in this process, a recording mark is formed at each place in a predetermined length. Thus, a predetermined signal or information is recorded in the recording layer, in the form of change in the magnetic direction.
In the technical field of magnetic discs, it is known that if the recording layer has a higher coercive force, then the magnetic domain formed in the recording layer will have a higher thermal stability, and it is easy to form a finer or narrower, stable magnetic domain. The finer is the smallest magnetic domain stably formable in the recording layer, the greater recording density achievable in the magnetic disc.
As described, when recording information in a magnetic disc, application of a recording magnetic field which is stronger than the coercive force of the recording layer is necessary in order to form recording marks appropriately. This leads to an idea that with increasingly stronger coercive force selected for the recording layer, the magnetic head should apply a stronger recording magnetic field. However, intensity of the recording magnetic field applicable by the magnetic head is restricted by such aspects as the structure and power consumption of the magnetic head.
Given this restriction, so called thermally assisted magnetic recording method is sometimes utilized in information recording in magnetic discs. When recording information in a magnetic disc by means of a thermally assisted magnetic recording method, a laser beam for example, from an optical head placed near the recording surface of the rotating magnetic disc locally heats the recording layer of the magnetic disc successively. The region in the recording layer whose temperature has been raised due to the heating now has a lower coercive force than the surrounding region in which temperature was not raised. Then, a magnetic head placed near the recording surface of the magnetic disc applies to the temperature-raised region a recording magnetic field which is stronger than the coercive force of the temperature-raised region of the recording layer, thereby magnetizing part the temperature-raised region in a predetermined magnetic direction. The magnetization is fixed as the magnetized area cools down. In the thermally assisted magnetic recording method, these steps form a plurality of recording marks (magnetic domains) each having a successive inversion pattern of the magnetic direction and a predetermined length in accordance with the recording signal, along a track which runs in a circumferential direction of the disc. In magnetic discs which utilize the thermally assisted magnetic recording method, there is no need for very much increasing the intensity of the recording magnetic field applied by the magnetic head even if the recording layer is given a strong coercive force for information holding or information playback, since recording of information is made by first heating the recording layer thereby lowering the coercive force of the area, and then recording the information by applying a recording magnetic field to the area. Thermally assisted magnetic discs such as the above are disclosed in the following Paten Document 1 and Paten Document 2 for example.                Paten Document 1: JP-A-6-243527        Paten Document 2: JP-A-2003-157502        
When recording information in a thermally assisted magnetic disc, an intended recording region in the recording layer is heated. As the temperature of the intended recording region increases, the temperature of the surrounding area also increases significantly. On the other hand, in the thermally assisted magnetic disc technology, the recording layer should have a high coercive force in view of high recording density whereas the recording layer must be heated more intensely at a time of information recording if the recording layer has a higher coercive force. However, in conventional thermally assisted magnetic discs, intense heating at the time of information recording can cause too much region of the recording layer to be heated above the predetermined temperature. This causes problems: For example, the recording track along which the recording marks are being formed successively becomes undesirably widened into the adjacent tracks. This and other problems can cause a cross-writing phenomenon, i.e. erasure or deterioration of the recording marks on the adjacent tracks. The cross-writing phenomenon is an obstruct to the goal of narrowing the track pitch, and is undesirable from the view point of high recording density. As described, there are difficulties in conventional thermally assisted magnetic discs (thermally assisted magnetic recording mediums) if their recording density is to be increased.